In recent years, liquid crystal display panels characterized by light weight and high accuracy came to be widely used as display panels for various devices including cellular phones. As the use of the liquid crystal display panels is diversified, the environment where they are used is becoming strict. Further, liquid crystal display cells are also required to be highly accurate and uniform with high qualities.
A liquid crystal sealing composition is used to seal liquid crystals encapsulated between transparent glass substrates or plastic substrates provided suitably with transparent electrodes and an orientation film important as members constituting a liquid crystal display panel, so that the liquid crystals will not leak to the outside. In this sealing composition, an epoxy resin-based the heat cured resin composition of one-component type is widely used.
Small liquid crystal panels including cellular phones and display panels for car navigation are used outdoors or in automobiles, thus requiring improvements in impact resistance and resistance to high temperature and high humidity, and the liquid crystal sealing composition also strongly requires improvements in adhesion and heat resistance.
Generally, techniques of improving the adhesion of the heat cured epoxy resin compositions involve adding a large amount of rubber-like components such as CTBN, ATBN and ether elastomers or epoxy resin components modified with the rubber-like components in order to improve stress relaxation and adhesiveness. In this case, however, the glass transition temperature (Tg) of the resulting cured product is low due to the influence of the Tg of the rubber-like components, thus making heat resistance insufficient.
To improve the heat resistance of the epoxy resin composition used in the liquid crystal sealing composition, there is also a method of mixing a large amount of fillers such as glass fibers, glass particles and amorphous silica. In doing so, heat resistance is improved, but the resulting cured product tends to be brittle to lower adhesiveness.
In a production site for liquid crystal display panels, there is a trend that the time in a heating bonding step is reduced in order to improve the productivity of more uniform and high-quality liquid crystal display panels. From the viewpoint of productivity, a system for siamese heating press bonding of a plurality of plates is recommended and widely practiced. The system for siamese heating press bonding of a plurality of plates is a system wherein plural pairs of liquid crystal cell-forming substrates, one substrate of which is coated with a liquid crystal sealing composition, are layered, pressed under vacuum and thermally bonded in a heating furnace to produce liquid crystal display panels. A sheet press heating bonding system wherein a pair of transparent substrates for liquid crystal cell is sealed one after another by heat press bonding.
For this sheet press bonding system, for example, JP-A 10-273644 proposes a liquid crystal sealing composition comprising (a) liquid epoxy resin, (b) a curing agent consisting of novolak resin having a softening point of 75° C. or less as determined by a ring and ball method, (c) a filler having a particle diameter of 10 μm or less and (d) a curing accelerator as essential ingredients, wherein a mixture of the components (a) and (b) is liquid or has a softening point of 50° C. or less as determined by a ring and ball method. When the epoxy resin component and the curing agent component are in the form of liquid or have such low softening point, the composition undergoes rapid reduction in viscosity in a the heat cured step in the sheet press heating bonding system, thus easily bringing about a phenomenon such as burnout in a seal form or foaming in the seal. Further, the glass transition temperature of the cured liquid crystal sealing composition tends to be lowered to deteriorate heat resistance.
JP-A 2001-64483 proposes a sealing composition for liquid crystal display panel, which comprises epoxy resin, a curing agent, epoxylated polybutadiene and an inorganic filler. In this publication, the epoxylated polybutadiene component is essential. This sealing composition is excellent in retention of seal shape in the sheet press heat bonding system, but the glass transition temperature of the epoxylated polybutadiene component is low, and thus the glass transition temperature of the cured liquid crystal sealing composition is also lowered, thus easily deteriorating heat resistance.
Accordingly, the object of the present invention is to provide a novel liquid crystal sealing composition which is compatible with the sheet press heating bonding system, can demonstrate high-speed production and is superior in cell adhesion, moisture permeation resistance and heat resistance.